1. Field of the Invention
This invention relates to code boards and, more particularly, to a code board having formed on one surface thereof a number of Gray code patterns being scanned by respective brushes or like reader members to give off a digital signal whose bits correspond to the patterns and alternate between two logic values.
2. Description of the Related Art
To convert analog input data into its equivalent digital form, there have been many previous proposals for utilizing a code board having Gray code patterns or the like in combination with a reader in the form of a brush for reading information representing the position of an object, as, for example, in U.S. Pat. No. 4,639,108 and Japanese Laid-Open Patent Application No. Sho 58-125188.
FIG. 3 illustrates a conventional code board 11 having a Gray code pattern portion composed of a plurality of electrically conductive and non-conductive patterns constituting part of the flexible printed circuit board 10. The Gray code pattern portion on the code board 11 is formed in five patterns P0, P1, P2, P3 and P4 from which respective bits of a digital signal are generated. The number of transitions over the entire length of each pattern becomes progressively smaller from the upper to the lower pattern or in the order of patterns P0, P1, P2, P3, P4. To read such a Gray code pattern portion, five sliders or brushes of a reader 12 are brought into registry with the respective patterns, as shown in FIG. 2(A).
In connection with each monotonic zone, a phenomenon takes place in reading the gray code pattern portion of FIG. 3 which is explained below. As the reader 12 moves rightward, a first zone 1 is partitioned by a transition line A in the Gray code pattern P0 and a transition line B in the Gray code pattern P1, a second zone 2 by the transition line B in the pattern P1 and the next transition line C in the pattern P0, a third zone 3 by the transition line C in the pattern P0 and a transition line D in the Gray code pattern P3, and a fourth zone 4 by the transition line D in the pattern P3 and the next transition line E in the pattern P0.
In such a manner, any of the monotonic zones is made to lie between one of the transition lines of the Gray code pattern P0 and one of the transition lines of the other patterns P1 to P4.
In order to read the Gray code patterns accurately with a given resolution, the perpendicularity of a line passing through the points at which the tips 12a of the brushes of the reader 12 come into contact with the Gray code patterns to the path of movement of the reader 12 must be sufficient so as to fall within the width of the monotonic zone.
In actual practice, however, the line of contact will be erroneously declined from the direction of the transition line to some angle, .theta., as shown in FIG. 2(A). In the same figure, 13 is a fastener screw for the reader 12, and 14 is a location pin.
FIG. 2(B) is a fragmentary view in exaggerated scale of parts of the Gray code pattern portion and the reader 12 of FIG. 2(A) which are near the tips 12a of the brushes across the Gray code patterns P0 to P4.
Now, as shown in FIG. 2(B), let the width of the same code zone in which the read values from the Gray code patterns are identical as the reader 12 moves, be denoted by l.sub.1 ; the position of the tip 12a of the first brush which traces the transition lines in the Gray code pattern P0 be denoted by X1; the position of the tip 12a of another brush which traces the transition lines in the Gray code pattern Pn (n.gtoreq.1) be denoted by Xn; the angle of inclination of the reader 12 with the transition line of the Gray code pattern be denoted by .theta..sub.1 ; and the tolerance for the relative positions of the tips 12a of the brushes to the common design line be denoted by 1. The read error d.sub.1 due to the inclination .theta..sub.1 of the reader 12 with the transition line of the Gray code pattern is given by the following expression: EQU d.sub.1 =.vertline.X1.multidot.Xn.vertline..multidot.tan .theta..sub.1 ( 1)
Hence, in order to convert the analog input data to digital form accurately with the desired resolution, it is required that the width l.sub.1 falls within the following range: EQU l.sub.1 &gt;.DELTA.1+d.sub.1 EQU that is, EQU l.sub.1 &gt;.DELTA..sub.1 +.vertline.X1.multidot.Xn.vertline..multidot.tan .theta..sub.1 ( 2)
However, in actual practice, it is very difficult to set up an operating mechanism so that the reader 12 runs over the code board while its path of movement is maintained exactly perpendicular to the transition lines, or at .theta..sub.1 =0. For this reason, the read error tends to be unavoidable.